This invention relates to the production of chemimechanical pulps from wood or other lignocellulosic materials, such as chips, shavings and sawdust, with ultra high yields and with improved strength properties. More particularly, this invention relates to the production of such pulps by means of the sulfonation of the lignin in the wood, using aqueous sulfite or bisulfite solutions, followed by mechanical defibering.
The pulp and paper and related industries use many processes to produce pulp from wood chips and other lignocellulosic materials. These processes can be classified, the purposes of discussion, into four groups, shown below with the representative yields:
Chemical Pulps -- up to 60% yield PA1 Semichemical Pulps -- 60-80% yield PA1 Chemimechanical Pulps -- 80-95% yield PA1 Mechanical Pulps -- at least 90% yield
The yield ranges shown are approximate only.
Chemical pulps are prepared by cooking the wood chips (or other lignocellulosic material) at elevated temperatures and pressures with various chemical agents which dissolve the lignin and some carbohydrate material to leave relatively pure cellulose fibers at the 40-45% yield level or cellulose plus some residual lignin at somewhat higher yield levels (45-55%).
Mechanical pulps at the other extreme use mechanical means such as grindstones to defiber logs or disc refiners to defiber wood chips into pulp. These processes use water for cooling and dilution purposes so that the appoximately 5% of the wood substance that is water soluble is lost for a net yield of about 95%.
Chemical pulps have many advantages due to their cleanliness, high strength, and ease of bleaching, but they are expensive to produce due to the low yield. Their dissolved solid and gaseous waste products give rise to many environmental problems.
Mechanical pulps are much cheaper to produce due to their high yield and constitute an efficient use of forest resources. Such processes offer no gaseous pollution and relatively little BOD.sub.5 (biochemical oxygen demand, 5-day test) discharge compared to chemical pulps.
The semichemical and chemimechanical pulping processes fall midway between the chemical and mechanical processes in these respects.
The increasing world-wide demands for pulp, paper and other forest resource based products are creating an increasing need for the use of higher yield pulps due to the decreasing availability of fiber. The present invention produces a high yield pulp that can replace some types of chemical or semichemical pulp in many products.
It is known that the treatment of wood chips with relatively small amounts of sulphite and bisulphite, at near neutral pH, and under relatively mild conditions (100.degree.-150.degree. C., for 2-15 minutes) produces a softening effect on the chips which makes them easier to defiber and generally produces a cleaner and better draining pulp than can be produced by mechanical means alone. See "Ultrahigh Yield NSCM Pulping", by C. A. Richardson, Tappi, Vol. 45, No. 12, pp. 139A-142A (1962); Richardson et al., "Supergroundwood from Aspen", Tappi, Vol. 48, No. 6, pp. 344-346 (1965); Chidester et al. "Chemimechanical Pulps from Various Softwoods and Hardwoods", Tappi, Vol. 43, No. 10, pp. 876-880 (1960); Uschmann U.S. Pat. No. 3,607,618; Aitken et al. U.S. Pat. No. 3,013,934; and Asplund et al. U.S. Pat. No. 3,558,428.
However, the pulps produced by such processes, while being superior to conventional mechanical pulps in terms of cleanliness and drainage properties, do not have sufficiently good physical properties to justify their increased cost of production relative to the conventional mechanical pulps.
Better properties can be achieved by cooking under more severe conditions such as increased temperatures in the 160.degree.-240.degree. C. range, but the strenght improvement is always accompanied by a loss in yield. Instead of yields of over 90%, the yields are reduced to about 70-85%. See most of the above publications and patents and Richardson U.S. Pat. No. 2,962,412; Zimmerman U.S. Pat. No. 1,821,198; Cederquist U.S. Pat. No. 3,078,208; Asplund et al. U.S. Pat. No. 3,446,699; Von Hamzburg U.S. Pat. No. 2,949,395; Olson U.S. Pat. No. 3,003,909; and Risch et al. U.S. Pat. No. 2,847,304.
Considerations of cost and environmental protection make the maintenance of yields in excess of 90% highly desirable.
It is well known that the physical properties of wood pulps are strongly influenced by the flexibility of the individual fibers--which flexibility permits the fibers to be brought into closer contact with each other during the pressing stages of the paper-making process, which in turn leads to better bonding and improved strength. Natural wood fibers are rendered relatively inflexible by the presence of large amounts (20-30% by weight) of lignin which is a relatively rigid material at moderate temperatures (less than 100.degree. C.). Fiber flexibility is improved in conventional chemical or semichemical pulping processes by removing, chemically, at least part and in some cases nearly all of the lignin.
The present invention modifies the lignin by sulfonating it sufficiently to produce a marked change in the physical and chemical properties of the lignin, but not enough to render it soluble in water or in the cooking liquor, so it is not substantially removed from the wood fiber, and yields are consistent with those of purely mechanical pulps (90-95%).
Many softwood species such as spruce can be sulfonated up to about 0.65% (expressed as combined sulfur on wood and measured by the test method, below), usually without reducing the yield below 90%. Conventional high yield chemimechanical pulping processes such as those reported by the Richardson and Chidester et al. publications and Asplund et al. patent, supra, achieve a level of about 0.3 to 0.35% sulfur (on spruce) or only about 50% of the maximum level of sulfonation that can be reached without reducing the yield below about 90% (see comparative prior art Example 9 below). This low level of sulfonation achieves some softening of the lignin, which permits the chips to be more readily defibered than untreated chips, but the individual fibers so produced are still relatively stiff and do not give strong pulps. The stiffness of the fibers also makes them prone to damage (cutting) in the refining stages and the consequent production of fines and debris--although not to the same extent as untreated fibers.
It is, accordingly, an object of the present invention to provide a high yield chemimechanical process for producing pulp from wood chips and other lignocellulosic materials, including shavings and sawdust.
It is another object of the invention to provide a process for producing high yield chemimechanical pulp from wood chips whereby the chips are sulfonated to a high degree of sulfonation and thereafter readily defibered by customary mechanical means to provide a pulp having excellent strength characteristics.